In the field of graphic arts, an image is printed on a printing plate through a set of color separation films made from a color original. In general, a color proof is made from the color separation film before the regular printing. The color proof is then checked for errors in the color separation process, the necessity of color correction, etc. It is said that as materials for color proofs there are preferably used pigments as coloring materials as well as image-forming materials to obtain a high approximation to printed matters. Further, in recent years, a dry proof preparation method free of developer has been desired in combination with a high resolving power enabling a high reproducibility of halftone images and a high process stability.
As a method for correction of color prints there has heretofore been used a method which comprises using a printing correcting machine to make a proof for color correction. However, this method consumes time and requires labor and a great deal of skill. Further, this method has a limited reliability.
On the other hand, as easier and simpler proof methods various photographic proof methods have been developed to replace this proof method. Among these photographic color proof methods, a color proof method using a photopolymer (prepress proof) is widely used.
A photopolymerizable system is a typical example of such a photopolymer system. Conventional processes for use with photopolymerizable systems involve exposure through a photographic intermediate film as a mask, and many such methods have been put into practical use as described hereinafter. In order to obtain a high image quality color proof, it is normally necessary to reproduce a halftone image having 150 lines per inch. This method essentially provides a high resolving power. Most initiators for the conventional photopolymerization system are sensitive to the near ultraviolet range. On the other hand, as disclosed in U.S. Pat. No. 4,162,162 many spectral sensitizers which are sensitive up to the visible light range have heretofore been developed. Thus, it has become relatively easy to realize a high recording sensitivity to lasers in the visible light range such as argon ion lasers and helium-cadmium lasers.
The recent spread of electronized systems in the prepress process and the requirement to shorten the process time and reduce the amounts used of expendables such as films have caused an increasing demand for recording materials and systems for preparing a color proof directly from a digital image signal without going through an intermediate material such as lith film. It is thus desired to use as a recording head a laser which can be modulated by a digital signal and can be converged to a fine recording beam to record a high image quality proof from the digital signal. To this end, the recording material requires a high resolving power enabling halftone reproduction as well as a high recording sensitivity to a laser beam.
However, an essential problem is that materials have not yet been developed which can provide direct recording of a digital signal and can be put into practical use.
The above mentioned photopolymerizable system has a high potential possibility of being used for digital image recording from the standpoint of resolving power and laser recording sensitivity, but nothing practicable has yet been obtained. The conventional photographic multi-color image recording technique using a photopolymerizable material will be outlined below, and its problems against the objects of the present invention will also be discussed below.
Conventional photographic methods of correcting color prints include the overlay process and the surprint process. In the overlay process, a plurality of color proofing sheets comprising various color separation images formed on transparent supports are prepared. These color proofing sheets are laminated for color proofing (such a laminate is called a "color test sheet").
The overlay process is simple and inexpensive and has the advantage that the lamination of only two or three colors allows a continuous check. However, the overlay process is disadvantageous in that the lamination of a plurality of synthetic resin sheets gives a slightly dark color test sheet and in that the incident light beam reflects from some of these sheets, making a big difference between the impression on the color test sheet and that on printed matters prepared by printing machines.
On the other hand, in the surprint process, several colored images are superimposed on a single support. To this end, various colored layers are applied to a single opaque base or corresponding toners are sequentially applied to the opaque base. This lamination process is advantageous in that the color density is not affected by the synthetic resin base. This process is also advantageous in that it is easy to use as a coloring material a pigment having the same or close to the same hue as the pigment used in printing ink. Thus, this process has been widely used.
As an example of the surprint process, there has a known process comprises imagewise exposing to light a light-sensitive transfer material comprising on a tentative support a release layer made of an organic polymer, a coloring material layer and a light-sensitive layer sequentially laminated thereon, wet-developing the material to form a colored image on the release layer, and then transferring the colored image with the release layer to any support (permanent support) using an adhesive (JP-B-46-15326 and U.S. Pat. No. 3,721,557). This process can be advantageously used as a color proofing process for various operations such as overlay type and surprint type operations. However, this process is disadvantageous in that various colors are sequentially transferred on a paper support having a poor dimensional stability, making it difficult to maintain a high precision of registration of the various colors. Further, the image thus obtained has a poor mechanical strength.
As an approach for eliminating these difficulties, a process which comprises the transfer of an image to a tentative image receiving sheet (image receiving element) before the transfer of the image to a permanent support is described in U.S. Pat. No. 4,482,625 to the present inventors. That is, this process comprises the steps of preparing a tentative image receiving element comprising an image receiving layer made of a photopolymerizable material on a support, transferring various color images to the tentative image receiving element before the transfer to a permanent support, retransferring the images to the permanent support, and then entirely exposing the material to light so that the photopolymerizable image receiving layer thus transferred is cured. In this process, the registration and transfer of the various color images can be effected on a support having a high dimensional stability such as polyethylene terephthalate. This gives improvements in the precision of registration as well as the quality stability. Further, the photo-set layer having a great hardness protects the final image, rendering the mechanical strength of the image excellent.
However, this process theoretically has an essential problem of wet development. Further, from the standpoint of adaptability to laser recording, the use of a dye sensitizer spectrally sensitive to radiation from the visible light range to the infrared range theoretically enables laser recording. However, since the light-sensitive layer and the coloring material layer are the same layer or adjacent to each other, both of these two layers are transferred to the image receiving layer. The hue of the resulting image is greatly changed due to the colored sensitizer, making it impossible to put this process into practical use.
Many multi-color image formation processes which eliminate the above mentioned disadvantages of wet development processes have been heretofore proposed and partly put into practical use. Examples of these multicolor image formation processes are described in U.S. Pat. No. 3,060,024, 3,582,327, and 3,620,726. In these processes, an adhesive photopolymerizable copying material comprising a support layer and a photopolymerizable layer containing at least one addition-polymerizable monomer and a photopolymerization initiator is cured upon imagewise exposure to light to lose its adhesivity. The resulting latent image can be visuallized by the addition of a proper powdered toner material. This toner is attached to only the unexposed adhesive region. The toner can be removed from the exposed nonadhesive image region. This process is a dry development process requiring no developer. Because of its simplicity and convenience, this process is widely used in proofs using a color separation film.
However, this process is disadvantageous in that the powdered toner can contaminate the working atmosphere and there can easily become a difference in the way of applying/removing the powder toner among individuals. Thus, this process has been keenly desired to be improved.
As an approach for improving the above mentioned process using an adhesive photopolymerizable light-sensitive layer, the use of a toner in the form of film is disclosed in U.S. Pat. No. 4,806,451 and JP-A-2-14985 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). In this process, as in the above mentioned process, a light-sensitive layer which is adhesive when not exposed to light but loses its adhesitivity upon exposure/curing comprises a photopolymerizable monomer and a photopolymerization initiator. The light-sensitive layer is laminated on a receiver base such as paper together with a support. The latent image which has been exposed upon imagewise exposure and peeling of the light-sensitive layer support is then brought into contact with a film toner provided on a separate support. The toner is transferred to the latent image under heating and/or pressure for development. Among the above two cited patent applications, the former is characterized by mixing an incompatible polymer as a binder in the toner layer while the latter is intended to improve the image quality by properly selecting the thermal and mechanical properties of the binder polymer to be contained in the toner layer. Both of the two processes are intended to eliminate the disadvantages of the above mentioned process with a powdered toner. These processes provide an excellent approximation to printed matters (pigment image formation on a regular printing paper) and enable dry development. These processes provide improvements in the quality stability. Thus, these processes are considered remarkably desirable as compared with the conventional processes.
However, the use of such a film toner leaves much to be desired.
Firstly, since the registration of imagewise exposure of the light-sensitive layer is repeated on a paper support having a poor dimensional stability, the various color images can easily deviate from each other in position. Further, the final image has a low mechanical strength. In order to render the final image scuff-resistant and adhesion-resistant, it is necessary to add a complicated process such as lamination of a protective layer on the final image.
Moreover, this process gives essential restrictions on the object of the present invention in which a high image quality color proof is obtained directly from a digital image signal without going through an intermediate film.
Using the process described in the above cited U.S. Pat. Nos. 4,806,451 and 4,935,331, laser recording can be effected merely by combining the photopolymerization initiator described in the above cited JP-A-54-155292 with a visible light from an argon laser. However, since photopolymerization initiators sensitive to the visible light range or longer wavelength ranges exhibit a strong absorption in the visible light range, a photopolymerizable light-sensitive layer comprising these initiators cannot be prevented from being colored. Accordingly, if laser recording and image formation are effected in accordance with the process disclosed in these patent applications, the resulting image is stained by colors developed by the initiator, and the portion which should be essentially colorless is heavily colored, making it difficult to put this process into practical use.